1. Field of the Invention
This invention relates to optical transmission systems, and more specifically to noise reduction in fiber optic transmission systems carrying broadband signals.
2. Description of the Related Art
Fiber optical transmission is commonly used for the economical distribution of broadband signal content (such as multi-channel cable television (CATV) systems) as a means to transmit amplitude modulated/frequency division multiplexed (AM/FDM) signals over long distances, with or without repeaters. Such optical fiber links apply optical intensity modulation to a light source, sending the modulated light signal via fiber to a receiver which converts the optical intensity signal back to an electrical signal. The modulating electrical signal is a composite signal with the AM video channels multiplexed in frequency.
Externally modulated differential detection transmission systems, such as the systems described in U.S. Pat. No. 5,253,309, entitled "Optical Distribution of Analog and Digital Signals Using Optical Modulators with Complementary Outputs," by Nazarathy et al., and incorporated herein by reference in its entirety, are known to reduce two signal degrading effects associated with the above described optical transmission, namely, phase-to-amplitude noise conversion and fiber-induced composite second order distortion (CSO). In addition, the use of differential detection raises the net carrier-to-noise ratio (CNR) by taking advantage of complementary carrier power that would otherwise be lost at the optical modulator.
FIG. 1 illustrates a prior art externally modulated differential detection transmission system 100. Optical source 110, typically a laser such as a distributed feedback laser (DFB), provides an optical carrier to modulator 120, typically a dualoutput lithium niobate (LiNbO.sub.3) electro-optic modulator. When properly biased by bias voltage 122, modulator 120 will produce two lightwave signals (e.g., complementary output signals 130 and 140), encoded with the same RF information (as provided by RF input 124), and 180.degree. out of phase with each other. These signals can propagate through optical media such as optical fibers 150, and subsequently be detected by a balanced receiver 160 which includes, for example, two distinct photodetectors. The detected signals can then be recombined in the RF domain to provide RF output 170. As long as the total path lengths of the two transmission links (including both the optical fiber and coaxial cable elements of those paths) are within (c/n)/.nu. of each other, where v is the highest frequency CATV channel and c/n is the speed of light in the fiber, the RF carriers will add coherently, generally increasing by 6 dB over the carrier power for a single transmission link. Additionally, most of the noise from the two photodetectors will add incoherently, thereby increasing by 3 dB. The net benefit is a 3 dB increase in the CNR. Common mode noise and distortion will cancel in the recombined signal.
Three examples of common mode noise and distortion canceled by differential detection are relative intensity noise (RIN) associated with the DFB optical source, phase-to-amplitude noise conversion, and CSO. Since the complementary output signals 130 and 140 of modulator 120 are derived from the same optical source, the RIN caused by intensity fluctuations in the DFB is common to both output signals, and appears with equal amplitudes and in the same direction with respect to the quadrature point to which the modulator is linearized. Thus, upon subtraction of the two detected signals at balanced receiver 160, the RIN is canceled. In contrast, the carrier signals in the two complementary output signals 130 and 140 have equal amplitudes but opposite directions with respect to the quadrature point to which the modulator is linearized. Consequently, subtraction of the two signals at the balanced receiver results in reinforcing the resultant signal to double the value of the modulation signals in each transmission link.
CSO is generated by the interaction of self-phase modulation with dispersion in the optical fiber. The effect generally increases as the square of the fiber length, dispersion, and launch power, and is most severe at high channel frequencies. The use of differential detection both cancels the fiber-induced CSO and minimizes the effect of the external phase modulation on the CSO.
Dispersion in the optical fiber will convert phase noise into amplitude noise. This effect is most severe at high channel frequencies. At high frequencies (e.g., greater than 400 MHz) differential detection provides additional CNR improvement of approximately 1.5 dB beyond what would be expected for uncorrelated noise. This effect is due to the cancellation of phase-to-amplitude noise, which is correlated between the two fiber links because it originates in the noise of the common DFB laser.
Balanced receiver 160 can be implemented in a variety of ways, as illustrated by the examples shown in FIGS. 3A-3B. In FIG. 3A, complementary output signals 130 and 140 are optically coupled to identical photodiodes 310 and 320, respectively. Photodiodes 310 and 320 convert the incoming optical signals into photocurrents. For the RF signals typically produced by photodiodes 310 and 320, capacitors 330 and 340 essentially provide a short circuit and inductor 350 essentially provides an open circuit. Reactive impedance matching circuit 360 is then used to extract a differential RF output signal 170. A balanced receiver may also be realized by taking a the RF outputs from pair of standard optical receivers modules, applying a phase shift of 180.degree. degrees to one RF leg, then combining the resulting RF signal in phase.
In FIG. 3B, complementary output signals 130 and 140 are optically coupled to identical photodiodes 315 and 325, respectively. Photodiodes 315 and 325 convert the incoming optical signals into photocurrents. These photocurrents are amplified by amplifiers 370 and 380. Differential amplifier 390 is used to subtract the amplified signals from each other and thereby produce RF output signal 170.
As illustrated by FIG. 1, many externally modulated differential detection transmission systems do not need additional optical amplification. However, when the length of the fiber link is extended, or when other optical power losses must be compensated for (e.g., insertion losses at the external modulator) optical amplifiers are often used to amplify the optical signal. The availability of efficient erbium doped fiber amplifiers (EDFAs) operating in the 1550 nm wavelength region, where standard telecommunication fiber exhibits its minimum attenuation, has motivated the development of broadband transmitters compatible with the gain bandwidth of EDFAs. FIG. 2 illustrates a transmission system 200 (similar to transmission system 100 of FIG. 1) that includes EDFAs 210. The optical power is amplified by an EDFA which is downstream from the external modulator. Thus the information bearing light signal enters the fiber optic span with an optical signal determined by the saturated output power of the EDFA. Note that additional EDFAs 220 and fiber lengths 230 can be added to transmission system 200 to extend its overall length.
However, a closer look at the CNR of such a transmission system demonstrates that EDFAs added to the system contribute additional noise. CNR is a function of the optical modulation index m, the photocurrent at the receiver I.sub.photo, electrical bandwidth B.sub.e (typically 4 MHz for NTSC based CATV systems), shot noise at the receiver 2eI.sub.photo, thermal equivalent noise i.sub.th, and the RIN of the signal: ##EQU1##
Note that in equation 1, RIN is the sum of RIN from a variety of sources including the DFB laser. For example, each EDFA inserted into the transmission system contributes: ##EQU2##
where NF is the EDFAs noise figure, P.sub.in is the optical input power to the EDFA, h.nu. is the photon energy, and G is the net gain of the EDFA.
CNR is a primary figure of merit in optical transmission systems, and thus raising the CNR for a given system is desirable. Accordingly, it is desirable to have an optical transmission system that includes optical amplifiers, while reducing some of the noise contributed by those amplifiers.